The general aim of the proposed research is to study the influence of the descending visual corticotectal connections on the functional development of the rabbit superior colliculus. The question of how normal development is effected is of central importance in developmental neurobiology. The capacity for plasticity, or the formation of abnormal connections, in addition, can be assessed adequately only when the normal process of development of the system in question is known. The normal timing and the sequence of appearance of functional properties of single cells in all of the major central visual areas in the rabbit nervous system have been described in detail. Visual deprivation disrupts the development of one class of receptive fields, those sensitive to orientation, in both the visual cortex and the superior colliculus. Several lines of evidence support the hypthesis that the loss of these fields in the superior colliculus is secondary to the cortical defect. The proposed research is designed to test this hypothesis. Development of the orientation-sensitive receptive fields in the superior colliculus following neonatal ablation of visual cortex will be studied electrophysiologically. The effect of inactivation of the descending corticotectal will be examined directly using a cortical cooling technique. Finally, the normal development of the corticotectal pathway will be charted with a variety of neuroanatomical techniques, and the possible development of aberrant crossed corticotectal connections following unilateral neonatal visual cortex ablation will be investigated.